Add reminder notifications to schedules

- ScheduleDocument/Schedule gain optional reminderMinutes (minutes from
  midnight; decode-compatible, no schema bump)
- New Workout form gets a Reminder section (toggle + time picker, hidden
  for Now; footer warns when notifications are denied)
- ReminderPlanner (pure, tested) derives notification triggers: daily
  repeating, per-weekday repeating for fixed days, one-shot for once
- ReminderScheduler resyncs pending requests from the cache on every
  change, multiplexed onto onCacheChanged after the watch push; asks
  notification permission only once a reminder actually exists

Shared/Model/Documents.swift

@@ -377,6 +377,10 @@ struct ScheduleDocument: Codable, Sendable, Equatable, Identifiable {
     var recurrence: String    // ScheduleRecurrence raw value
     var weekdays: [Int]?      // Calendar weekday numbers 1 (Sun) – 7 (Sat); .fixedDays only
     var date: Date? = nil     // the one-off day; .once only
+    /// Reminder time as minutes from midnight (local wall-clock, 0–1439); nil = no
+    /// reminder. Optional addition — decode-compatible, deliberately NOT schema-bumped
+    /// (an older app dropping it on rewrite loses only a notification preference).
+    var reminderMinutes: Int? = nil
     var order: Int
     var createdAt: Date
     var updatedAt: Date